This invention relates to a method for the low-transient power control of electrical loads, particularly temperature-dependent loads which are each electrically divided into essentially equal sub-loads, the sub-loads of each load being connectable to an a.c. mains in order to receive power.
Such a method is known from EP 303 314 B1. In this known method the sub-loads of each load can be connected to an a.c. power mains selectively in at least three main power stages in series, alternately separately or in parallel. This is effected by means of three triac control devices which are connected in a bridge arrangement which can be controlled by means of a control unit.
In such methods care must be taken that the mains reaction (i.e., switching surges) during power control of the electrical loads does not exceed a maximum permissible value. The maximum permissible values for mains reaction are defined in IEC-1000-3-3: Electro Magnetic Compatibility (EMC), Part 3, Limits-Section 3: Limitation of voltage fluctuations and flicker in low-voltage supply systems for equipment with rated currents &lt;16A.
According to this standard the starting currents are limited in that the maximum permissible relative voltage fluctuations of the supply voltage during one mains half-wave are defined. On the other hand, the maximum permissible mains reaction produced when the loads are turned on and turned off periodically are defined. The mains reaction produced as a result of periodically turning on and turning off the loads is referred to as flickering. For small power levels the standard is difficult to meet in the case of temperature-dependent loads (for example, halogen lamps) when the power rise between turn-off and turn-on increases as result of cooling of the loads.
In the method described in EP 303 314 B1 the two sub-loads are basically arranged in series for the smallest power levels. Three switching elements are employed for controlling the power. If the switching elements are realized by means of semiconductor devices, such as for example, triacs, two of the devices do not have a common operating level and must be driven in an intricate manner via optocouplers or ignition transformers. Moreover, there is a risk of short-circuit owing to misfiring of the switching elements themselves. If mechanical relays are used as the switching elements for changing over between series/parallel connection, this has the disadvantage that switching noises are produced in a medium-power range. Furthermore, an additional device (relay or triac) is needed.